Some examples:
- different endings for verbs
- Polish different perfekt (Polish is unique in that amongst other Slavic languages)
- where Czech and Slovak has "h" Polish (like almost all other Slavic languages) has "g" (ok not in all cases, but mainly - ethimologically, CZ and SK "h" is other's "g")
- Slovak has more words similar to Polish than Czech (to Polish), while Czech and Slovak are the most similar
- different conugation
- and so on....

More or less, same as if you ask between e.g. Czech, Croatian and Slovene or Polish, Ukrainian and Russian and so on...

While this in theory is true I'd see this rather as an advantage of Slovak and Polish over Czech.
(Only kidding of course, as supposedly even Czech children have troubles learning the correct pronunciation of "ř". )

Well, in all earnest:

- Slovak has a great many diphtongs, as has Polish and Croatian plus Slovene dialects;
- Slovak has a dark back vowel where Czech (and generally all Slavic languages) haven't: "som" for example, vs. CZ "jsem" and PL "jestem" (Slovene "sem", BCS "sam" etc.; this feature of Slovak sounds quite exceptional to me and I've wondered if it could be due to Hungarian influence;
- Polish phonology significantly differs from Czech and Slavic phonology; the latter are rather similar despite all those diphtongs and the dark vowels ("som") plus a few other differencies in phonology;
- I'd say lior neith's example shows quite well that Slovak and Czech are also rather close concerning grammar and idioms while Polish differs here significantly.

And so on.
Of course, all three - PL, CZ and SVK - really are quite close, especially if you compare at a dialectal level.
We've had that somewhere in this forum before, I can't quite remember where - that Eastern Slovak dialects supposedly have an accentuation which is somewhere in-between Standard Slovak and Polish accentuation.

(The question is a little pointless really because it's way too broad - to compare at such a level, including all and everything, really would require an entire book; a thread won't do. )

The West Slavic languages are divided into 3 subgroups: Czech-Slovak, Lechitic and Sorbian, where Czech and Slovak fall under the Czech-Slovak group and Polish under the Lechitic one. Czechs and Slovaks can generally talk to each other and understand fully (most of the time) what the other is saying while a Czech or a Slovak speaking Czech/Slovak to a Pole can usually only make himself understood just to get along, rather than involve in meaningful conversations (especially when one doesn't speak any foreign language, otherwise, at least from personal experience, English is preferred).

Slovak has a more regular grammar than Czech.
Slovak uses palatalization more often.
Czech and Slovak have different declension and conjugation endings.
Czech, unlike Slovak, uses the vocative.
Slovak uses the passive voice formed like in English less than Czech.
There's a "rhythmical shortening rule" (rytmické krátenie) in Slovak according to which there cannot be 2 long syllables in a row in one word. Diphtongs are regarded as long syllables as well (for example SK -biely, CZ -bílý). There are some exceptions, though.

And phonetic differences of course, both languages have rather distinct pronunciation and intonation.

But the ending still remains removable, as if it actually were a separate word, attached to the participle. So you could say "ty to zrobiłeś" but also "tyś to zrobił" or "toś ty zrobił", with no differencing in meaning (though emphasis is put on the word to which -ś is actually attached).

But the ending still remains removable, as if it actually were a separate word, attached to the participle. So you could say "ty to zrobiłeś" but also "tyś to zrobił" or "toś ty zrobił", with no differencing in meaning (though emphasis is put on the word to which -ś is actually attached).

As far as I know, all other Slavic lngs make perfect with auxiliary verb and main verb in past.
Polish goes only with sufix.
...
Polish: byłem / byłam; byłeś / byłaś; był / była; byliśmy; byliście; byli

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Well, Russian too, only personal pronoun + participle is used but no auxiliary - and I guess Belorussian and Ukrainian too.
And in Czech the auxiliary is not used in third person - see this previous discussion on Czech auxiliary.

That just for clarification; apart from that you're of course right, Polish still stands out among Western Slavic

Well, Russian too, only personal pronoun + participle is used but no auxiliary - and I guess Belorussian and Ukrainian too.
And in Czech the auxiliary is not used in third person - see this previous discussion on Czech auxiliary.

That just for clarification; apart from that you're of course right, Polish still stands out among Western Slavic

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Correct, Russian also don't use auxiliary (for Ukrainian and Belorusian don't know) and Czech and Slovak (as I write in examples) don't have it in 3rd persons, but still all of them have main verb in past without any sufixes or anything.
But there's no need to discuss that as we know how it function in each of those languages.

One other minor comment on the 3rd person auxiliary verb - while Polish merged the auxiliary verb with the participle to form one word, you can actually still see it explicitly in the old-fashioned, but still sometimes used and commonly understood past perfect tense - "zrobił był". Here past tense of "być" used as an auxiliary verb is still present in all persons.

Nice remark, though I would rather say from my nearer point of view, it is fifty-fifty in the aggregate.
A matter of course, this is not determining for Polish understandableness.
(To boot, excellent Czecho-Slovak mutual intelligibility may help both of them to "wider" to perceive Slavic languages pan-domain.)

- Slovak has a dark back vowel where Czech (and generally all Slavic languages) haven't: "som" for example, vs. CZ "jsem" and PL "jestem" (Slovene "sem", BCS "sam" etc.; this feature of Slovak sounds quite exceptional to me and I've wondered if it could be due to Hungarian influence;

There's a "rhythmical shortening rule" (rytmické krátenie) in Slovak according to which there cannot be 2 long syllables in a row in one word. Diphtongs are regarded as long syllables as well (for example SK -biely, CZ -bílý). There are some exceptions, though.

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In a matter of fact, the exceptions are not so unusual.Niečie skália riešia líščie dianie vtáčích básní súdiac býčie siatie vôní pávích piesní.
Naturally, Czech words with three long syllables in a row in one word (doznívánízůstává) are unavailable in Slovak language.

In a matter of fact, the exceptions are not so unusual.Niečie skália riešia líščie dianie vtáčích básní súdiac býčie siatie vôní pávích piesní.

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Speaking of unusual exceptions, this sentence is a gross nonsense, hence not the best example. The exceptions to this rule apply mostly to case endings, some compound words and some words with particular prefixes and suffixes.

Compared to Czech and Slovene standard plus BCS standard languages, yes it has.
Slovene dialects however have both "ie/ei" and "uo/ou" diphtongs, and this indeed reminds me of Slovak diphtongs (with "ie" and "ô").
And some Croatian dialects (from Kajkavian group) are similarily rich in diphtongs.

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Slovenian doesn't have diphthongs only in dialects, but also in the standard language. They can usually be found where the "l" is pronounced like "u", for example the past participle of verbs: delal (worked), where the final "l" is pronounced like "u", but it is pronounced together with a like a diphthong, so you have the final diphthong "au".

1) there is no ľ sound (ľ → l)
2) vocals l and r have only short variants (ŕ → r; ĺ → lou)
3) original Slavic g sound is extinct (g → k, zg → z, zk), only exceptionally k is assimilated to g sound (e.g. kdo /gdo/)
4) there is the exclusively Czech ř sound (ř → r)
5) ť and ň sound is less frequent at the end of the words (ť - > t, ť, ň -> n, ň; vlast × vlasť, pramen × prameň)
6) j sound is more frequently added at the beginning or end of the word (jaký × aký, jistý × istý, jsem × som, umyj × umy)
7) f sound is used only in words cognate to doufat and zoufat
8) the c and č sounds have no voiced counterpart, except of rare examples of assimilation (léčba /lédžba/, leckdy /ledzgdy/). Dž sound is rare (džbán), dz sound is extinct (dz -> z).
9) glottal stop is extensively used to separate two following vowels and to separate the words starting with vowel from preceding word
10) syllabic consonants are less frequent, but unlike in Slovak they could be at the end of the word and even m could be syllabic (švagr × švagor, vítr × vietor, metr × meter, sedm × sedem)

In Slovak:

1) there applies the rhytmic law
2) the letter v is pronounced as /ʊ/ at the end of closed syllables

Alternation

Not considering the Slovak rhytmic law, Czech and Slovak vowels alternate in quantity to the similar extent and mostly, by far not always, in the same words.

Slovak always tends to use the regressive assimilation. Czech mostly tends to the regressive assimilation, but for the sh cluster it uses the progressive assimilation (albeit regressive assimilation is acceptable as Moravian variant) and it tends to use no assimilation in front of sonors (jsme /sme/ × sme /zme/).

Nice remark, though I would rather say from my nearer point of view, it is fifty-fifty in the aggregate.

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I think slavic_one is right. Slovak shares practically all its vocabulary of Slavic origin with some of the other Slavic languages, but in Czech there is relatively big set of exclusively Czech (or Czech and Upper Sorbian) words of Slavic origin. This makes Czech less intelligible, in terms of vocabulary, than Slovak. On the other hand, this set is small in comparison with the set of words exclusive for Czech and Slovak.

Only some of the oldest generation pronounce it differently. That sound is predestined to disappear. Now, for most of the Slovaks it is only source of orthographic confusion. I think the natural next step should be replacing the letter ä with e.

OK, I think the texts could be a little bit changed (and still would be totally understandable and would sound natural) so that Polish looks more like Czech and Slovak versions. With such longer texts it always heavily depends on the way the speaker forms sentences.

* In case it is far [to the shop]. Previous Polish text said In case the shop is really near (lit. we have only piece of road to the shop). That is absolutely not what Czech and Slovak texts say.
** It is very common to say spytać się in Polish, however it is considered incorrect (correct form is without się, just spytać). This is however standard way of saying to ask in Czech and Slovak (zeptat se, spýtať sa)

Bibax, your text definitely doesn’t sound natural (those čehos, čemsi, jest, bo… not actually commonly used in Czech anymore as I believe). Of course it nicely shows that my version of Polish would be understandable for Czechs but it doesn’t show how similar or different these two languages are. My point when changing Polish text (and slightly Czech one) was to show natural sounding version, that would be similar and more comparable.

My idea is that you need texts in related languages written in similar manner, using similar words and constructions, if available in all compared languages and in common use and with similar meaning. Otherwise, if you write text in one of those languages in different style, you can show that this language is totally different, where it actually isn’t – one could, writing two texts with same meaning but using two different styles, prove that Polish and Polish are two totally different and not understandable languages. That was my point in changing the text.

Another thing is that, in my opinion, the most similar language to Polish is not even West Slavic. IMO the most understandable language without learning for Poles is… Belorusian. Although it is East Slavic, it has enormous amount of polonisms, in vocabulary, in grammar and phonology is quite similar to Polish.

It is a hypothetical text written by a Pole learning Czech (in fact I quickly edited the silmeth's Polish text). The text doesn't sound natural but it is grammatically correct and all words are used in Czech (frequency is another question). Even the word papirosa exists in Czech (however it is a special kind of the cigarettes, with a long embouchure, popular in the former USSR). Cosi, čehosi, čemusi, ... (cos, čehos, čemus) is common, robit and bo are used in the Ostrava region (and popularized in Nohavica's songs).

It is natural that Poles have the worst position in the ability of mutual understanding amongst the West Slavs as Polish contains not a small number of features that occur only in Czech or only in Slovak. Not to mention the numbers of speakers.

where Czech and Slovak has "h" Polish (like almost all other Slavic languages) has "g" (ok not in all cases, but mainly - ethimologically, CZ and SK "h" is other's "g")

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Needless to say, the biggest "personero" of Slavic languages, which one use in plenty "h" is Ukrainian.

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Little bit off topic: 'h' in Ukrainian (as well as 'g' in Russian) is used on ALL places where 'g' or 'h' is used in other languages. Unlike in Czech/Slovak, where these letters are strictly distinguished and can't be mutually interchangeable in both write or pronunciation (like it is for example in Russian spoken by Ukrainians, where 'g' is pronunciated as 'h', e.g. Gagarin – Haharin).

Very expressive example is the word hygiene, which though is not a Slavic word, but it demonstrates the differences well. Notice also i/y in Czech/Slovak, where these letters have the same pronunciation but different grammar vs. Polish/Ukrainian/Russian:
Czech/Slovak: hygiena
Polish: higiena
Ukrainian: hihienaRussian: gigiena

My very subjective perception of Polish (as being Czech having only minimal knowledge of Polish) in comparison to Czech/Slovak is that written Polish looks quite anciently in a lot of aspects and it reminds old texts written in mid-age Czech:
- compounds (rz, dz...) and usage of 'w' instead 'v',
- words/word endings (jest, -ie...) and word order (as demonstrated in the example above of Polish text modified into Czech),
While pronunciation doesn't sound ancientally, it just uses "too much" of sibilants (ć/cz, dź/dż, ś/sz, ź/ż – č, dž, š, ž), so Polish sounds very brigh and "ringing".

There is also a lot of words which sound similar, but have opposite or completely different meaning in Polish vs Czech/Slovak, so some Polish sentences sound very funny for Czechs/Slovaks and vice versa. For example Remarque's novel All Quiet on the Western Front (in Czech Na západní frontě klid) can be freely translated to Polish as Na zachodzie nie ma nowin, which means There is no (news)paper in the toilet room. But this is just a Czech joke which demonstrates this fact in its extremes, the correct and official Polish translation is Na Zachodzie bez zmian, which still means freely No changes while being in the toilet room I also noticed a thread in some other Polish forum with funny translations of Czech sentences into Polish.

It seems that nobody has mentioned yet the major phonetic difference between Czech/Slovak and Polish: Czech and Slovak distinguish between long and short vowels while Polish does not. The stress in Polish is almost always at the penultimate syllable, unlike in Czech and Slovak.